1. Field of the Invention
This invention relates to new polyisocyanate compositions which contain in dissolved or dispersed form certain reversibly blocked catalysts for the isocyanate addition reaction and which only develop their catalytic activity under the effect of compounds containing isocyanate-reactive groups, to a process for producing the new polyisocyanate compositions, to their use as reactants for compounds containing isocyanate-reactive groups in the production of polyurethane plastics and to latent tin catalysts suitable for the production of the compositions.
2. Description of the Prior Art
In industrial polyurethane chemistry, for example in the production of polyurethane foams, elastomers, lacquers or adhesives, the catalysis of the isocyanate addition reactions is a crucial factor. A survey of the catalysts normally used and the mechanism on which their action is based can be found in A. Farkas and G. A. Mills, Advan. Catalysis, 13, 393 (1962), J. H. Saunders and K. C. Frisch, Polyurethanes, Part I, Wiley-Interscience, New York, 1962, Chapter VI and K. C. Frisch and L. P. Rumao, J. Macromol. Sci.-Revs. Macromol Chem., C5 (1), 103-105 (1970).
A variety of different catalysts and catalyst combinations is used in the production of polyurethane plastics. The catalysts most widely used on an industrial scale are tertiary amines and metal catalysts, particularly tin salts of organic carboxylic acids. Metal catalysts are highly active for the urethanization reaction, i.e. they bring about a rapid and quantitative reaction of the isocyanate group with the hydroxyl group of the polyether or polyester. By contrast, tertiary amines catalyze both the urethanization reaction and also, preferentially, the isocyanate-water reaction which is responsible, for example, for the rapid hardening of NCO-prepolymers (one-component systems in the surface protection or adhesive field) and for evolution of carbon dioxide in the foam.
The disadvantage of conventional catalysts is that they can only be mixed with the isocyanate component in their effective concentration during the processsing step because otherwise they do not remain stable in storage. The reason for this is that polyisocyanates, particularly prepolymers containing terminal isocyanate groups, show a tendency during storage towards discoloration, towards the formation of reaction products of relatively high viscosity and/or towards the formation of crystalline precipitates under the effect of light, air and/or moisture. These undesirable phenomena are accelerated by the catalysts normally used to such an extent that polyisocyanates containing catalysts of this type must be regarded as unstable.
Accordingly, the object of the present invention is chemically to derivatize the polyurethane catalysts normally used, particularly tertiary amines and tin carboxylates, in such a way that they are present in an inactive form in which they may be dissolved or dispersed in polyisocyanates, particularly isocyanate prepolymers, without affecting their stability in storage and from which they may be spontaneously released again during the processing step.
According to the invention, this object is achieved by the polyisocyanate compositions according to the invention, which are described in detail hereinafter, and by the process according to the invention for their production.